MYC Oncogene Contributions to Release of Cell Cycle Brakes
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Review MYC Oncogene Contributions to Release of Cell Cycle Brakes Lucía García-Gutiérrez 1,2, María Dolores Delgado 1 and Javier León 1* 1 Instituto de Biomedicina y Biotecnología de Cantabria (IBBTEC) CSIC-Universidad de Cantabria and Department of Biología Molecular, Universidad de Cantabria, 39011 Santander, Spain; [email protected] (M.D.D.) 2 Current address: Systems Biology Ireland, University College Dublin, Belfield, Dublin 4, Ireland; [email protected] (L.G-G) * Correspondence: [email protected]; Tel: +34-942-201952 Received: 24 February 2019; Accepted: 18 March 2019; Published: 22 March 2019 Abstract: Promotion of the cell cycle is a major oncogenic mechanism of the oncogene c-MYC (MYC). MYC promotes the cell cycle by not only activating or inducing cyclins and CDKs but also through the downregulation or the impairment of the activity of a set of proteins that act as cell- cycle brakes. This review is focused on the role of MYC as a cell-cycle brake releaser i.e., how MYC stimulates the cell cycle mainly through the functional inactivation of cell cycle inhibitors. MYC antagonizes the activities and/or the expression levels of p15, ARF, p21, and p27. The mechanism involved differs for each protein. p15 (encoded by CDKN2B) and p21 (CDKN1A) are repressed by MYC at the transcriptional level. In contrast, MYC activates ARF, which contributes to the apoptosis induced by high MYC levels. At least in some cells types, MYC inhibits the transcription of the p27 gene (CDKN1B) but also enhances p27’s degradation through the upregulation of components of ubiquitin ligases complexes. The effect of MYC on cell-cycle brakes also opens the possibility of antitumoral therapies based on synthetic lethal interactions involving MYC and CDKs, for which a series of inhibitors are being developed and tested in clinical trials Keywords: MYC; cell cycle; CDK inhibitors; p21; p27; p15; ARF 1. Introduction The oncogene c-MYC (referred to herein as MYC) was the first described gene that encoded for an oncogenic transcription factor with the ability to transform cells in culture. MYC is overexpressed by different mechanisms in 60–70% of human solid and hematopoietic tumors [1,2,3,4,5]. The MYC family of proteins is composed of three members: c-MYC, N-MYC, and L-MYC. The existence of multiple MYC family members with distinct expression patterns reflects different requirements of MYC during development and in the adult animal, which is consistent with the specific way each gene is deregulated in certain cancer types [6]. MYC is a transcription factor of the helix-loop-helix-leucine zipper (HLH-LZ) family that regulates the activation or repression of many target genes [7,8]. Regulation of transcription by MYC depends on the formation of heterodimeric complexes with MAX protein [9]. The MYC-MAX heterodimer is the active form, which binds to specific DNA sequences called E-boxes (canonical sequence CACGTG) in the regulatory regions of target genes. The MYC network (also known as the MAX-MLX network), includes other components of the HLH-LZ family such as the MXDs, MNT, MLX and others, with different functions in gene expression regulation upon binding to E-boxes in the DNA (for recent reviews see [10,11]). The number of MYC-binding sites revealed by genome-wide technologies ranks between 7000 and 15,000 in different models. Indeed, MYC is bound at one or more sites of the regulatory regions Genes 2019, 10, 244; doi:10.3390/genes10030244 www.mdpi.com/journal/genes Genes 2019, 10, 244 2 of 28 of 10–15% of human genes [6,7,8,12]. In agreement with the large number of MYC target genes, overexpression of MYC deregulates a series of biological functions such as cell-cycle progression, nucleotide biosynthesis, energy metabolism, protein synthesis and ribosome genesis, genomic maintenance, immortalization, and differentiation [1,7,13,14,15]. Such deregulation confers ample competitive advantages to the cell and contributes to the well-stablished role of MYC in a wide variety of cancers. MYC protein contains several domains that play important roles in MYC functions, as well as many residues susceptible of being modified, modulating MYC’s activity and stability [6]. MYC contains an unstructured N-terminal region, which includes two conserved regions known as MYC boxes (MB) (Figure 1). MBI and MBII are located within the transcriptional transactivation domain (TAD), essential for MYC transcriptional and cell-transforming activity. The MBII is crucial for the recruitment of MYC transactivation co-activators such as TRRAP, GCN5, TIP48, TIP49, TIP60, CBP/p300, as well as SKP2 [16,17,18]. The central region of MYC contains the MBIII, which has been shown to be important for transcriptional repression [19,20] and MBIV, needed for MYC transcriptional activity and MYC induced apoptosis [21]. The C-terminal region of MYC includes the basic, helix-loop-helix, and leucine zipper domains (b-HLH-LZ). Through the basic domain, MYC protein recognizes specific sequences and binds the DNA, while the HLH-LZ domain mediates the dimerization with its major partner MAX [9,22,23]. Activation or repression of MYC-regulated genes is mediated by its interaction with a variety of partner proteins, many of them involved in chromatin structure regulation (recently reviewed in [17,24]). The mechanism for MYC-mediated transactivation depends on the recruitment of complexes containing histone acetyltransferases (HATs) [7,8] (Figure 1a). TRRAP (Transformation- Transactivation domain Associated Protein) was originally isolated as a cofactor of MYC and recruited to most of the MYC target genes upon mitogen stimulation [25,26]. Two different TRRAP containing complexes possess GCN5 HAT activity. TRRAP-containing TIP60 complex consists of the TIP60 HAT, the ATPase/helicase motif containing cofactors TIP48 and TIP49 and the SWI/SNF related protein p400 ATPase. Both GCN5 and TIP60 acetylate histones at MYC target genes. Furthermore, CBP/p300 interacts with MYC mediating its acetylation, increasing MYC stability and stimulating MYC-transcriptional activation [17]. MYC is present at the promoter of nearly all active genes acting as an amplifier of the transcription already going on at those genes [27,28] although there is some selectivity on the genes regulated by MYC [29,30]. Different studies support the idea of MYC as a transcription amplifier because of its role regulating global transcriptional pause release [31]. The mechanism is not well known but the activating interaction of MYC with P-TEFb (positive transcription elongation factor b) likely plays an important role in it [32] (Figure 1a). Apart from transcriptional activation of gene expression, MYC also represses a great number of genes, many of them involved in processes such as the inhibition of cell-cycle progression and cell adhesion [33,34]. MYC represses transcription by interacting with other transcription factors and co- repressor complexes at the core promoter region of genes. So far, MYC has been reported to exert its repression activity by interacting mainly with SP1 and/or MIZ-1 (Figure 1b). These two transcription factors normally activate transcription. However, interaction with MYC switches them into transcriptional repressors mainly by displacing SP1 and MIZ-1 co-activators. For example, MIZ-1 recruitment of p300 can be antagonized by MYC [35,36]. Further, MYC represses transcription through SP1 by recruiting histone deacetylases (HDACs) [37]. SP1-SMAD complex has been found to be inactivated by MYC resulting in gene repression [38]. MYC also interacts with SIN3 [19] and with HDAC3 [20]. In this way, MYC recruits HDACs to the core promoter of several genes, resulting in transcriptional repression. The MYC-MIZ-1 complex can recruit the DNA methyltransferase DMNT3A to promoters, repressing transcription. This might be an efficient mechanism to repress CpG island promoters [39]. At least two of the genes known to be repressed by MYC through these mechanisms encode proteins involved in cell-cycle regulation: CDKN1A (p21CIP1) [40,41], CDKN2B (p15INK4B) [35,38,42]. Genes 2019, 10, 244 3 of 28 Figure 1. Oncogene c-MYC (MYC) structure and interaction complexes. (a) MYC structural domains are represented. MB, MYC boxes I-IV; TAD, transactivation domain; PEST, PEST sequence; NLS, Nuclear Location Signal. b, basic; HLH, Helix-Loop-Helix; LZ, Leucine Zipper. Through these domains, MYC interacts with different cofactors involved in transcriptional activation (in green) or repression (in red). MYC-MAX interaction is also indicated. (b) Transcriptional activation through MYC-associated complexes. Upper: MYC-MAX heterodimers bind E-box sequences and interact with co-activators such as TRRAP, GCN5 and others. These complexes mediate histone acetylation to transactivate MYC target genes. Middle: CBP/p300 also mediates MYC acetylation and increased stability. Bottom: BRD4 is a reader of acetylated histones and promotes the activity of P-TEFb complex, composed of CyclinT1 and CDK9. MYC interacts with P-TEFb, which phosphorylates the C-terminal domain of RNA polymerase II to trigger elongation. (c) Transcriptional repression through MYC-associated complexes. Upper: MYC interacts with MIZ-1, displacing coactivators with HAT activity such as CBP/p300. The MYC/MIZ-1 complex binds to Initiator element (Inr) sequences and recruits the DNA methyltransferase DNMT3A to repress transcription. Middle: SP1-SMAD complex is repressed by MYC. Recruitment of HDAC1 contribute to histone deacetylation nearby Inr sequences. Bottom: MYC also recruits HDAC3 to E-box sequences, reducing histone acetylation. We will review here the role of MYC as cell-cycle brake releaser i.e., how MYC stimulates cell cycle mainly through the repression of cell-cycle inhibitors (Figure 2). Cell-cycle progression is regulated by serine/threonine protein kinases composed by a catalytic subunit or CDK (cyclin- dependent protein kinase), and a regulatory subunit, the cyclin [43,44]. CDK1, 2, 4, and 6 and A, B, E, and D-type cyclins constitute the major regulators of the mammalian cell cycle.